Pressure activated safety valve with high flow slit

ABSTRACT

A pressure actuated valve for controlling the flow of fluid through a medical device, the valve comprises a housing including a lumen extending therethrough and a flow control membrane extending across the lumen to control the flow of fluid through the lumen. The membrane includes a plurality of slits extending therethrough so that, when the membrane is acted upon by a pressure of at least a threshold magnitude, the slits open to permit flow through the lumen and, when not acted upon by a pressure of at least the predetermined magnitude, the slits are maintained closed by a biasing force applied thereto by the membrane to prevent flow through the lumen. Each of the slits extends between end portions thereof along a curve wherein a distance between a first end portion of a first one of the slits and a first end portion of a second one of the slits is a minimum distance between the first and second slits.

The present application incorporates by reference the entire disclosureof (1) U.S. application Ser. No. 10/768,571 entitled “Pressure ActivatedSafety Valve With Anti-Adherent Coating” filed on Jan. 29, 2004 namingKarla Weaver and Paul DiCarlo as inventors; (2) U.S. application Ser.No. 10/768,571 entitled “Pressure Activated Safety Valve WithAnti-Adherent Coating” filed on Jan. 29, 2004 naming Karla Weaver andPaul DiCarlo as inventors; (3) U.S. application Ser. No. 10/768,855entitled “Pressure Actuated Safety Valve With Spiral Flow Membrane”filed on Jan. 29, 2004 naming Karla Weaver and Paul DiCarlo asinventors; and (4) U.S. application Ser. No. 10/768,479 entitled “DualWell Port Device” filed on Jan. 29, 2004 naming Katie Daly, KristianDiMatteo and Eric Houde as inventors.

BACKGROUND OF THE INVENTION

Many medical procedures require repeated and prolonged access to apatient's vascular system. For example, during dialysis treatment bloodmay be removed from the body for external filtering and purification, tomake up for the inability of the patient's kidneys to carry out thatfunction. In this process, the patient's venous blood is extracted,processed in a dialysis machine and returned to the patient. Thedialysis machine purifies the blood by diffusing harmful compoundsthrough membranes, and may add to the blood therapeutic agents,nutrients etc., as required before returning it to the patient's body.Typically the blood is extracted from a source vein (e.g., the venacava) through a catheter sutured to the skin with a distal needle of thecatheter penetrating the source vein.

It is impractical and dangerous to insert and remove the catheter foreach dialysis session. Thus, the needle and catheter are generallyimplanted semi permanently with a distal portion of the assemblyremaining within the patient in contact with the vascular system while aproximal portion of the catheter remains external to the patient's body.The proximal end is sealed after each dialysis session has beencompleted to prevent blood loss and infections. However, even smallamounts of blood oozing into the proximal end of the catheter may bedangerous as thrombi can form therein due to coagulation. These thrombimay then be introduced into the patient's vascular system when bloodflows from the dialysis machine through the catheter in a later session.

A common method of sealing the catheter after a dialysis session is toshut the catheter with a simple clamp. This method is oftenunsatisfactory because the repeated application of the clamp may weakenthe walls of the catheter due to the stress placed on the walls at asingle point. In addition, the pinched area of the catheter may not becompletely sealed allowing air to enter the catheter which may coagulateany blood present within the catheter. Alternatively, valves have beenused at the opening of the catheter in an attempt to prevent leakingthrough the catheter when the dialysis machine is disconnected. However,the unreliability of conventional valves has rendered themunsatisfactory for extended use.

SUMMARY OF THE INVENTION

In one aspect, the present invention is directed to a pressure actuatedvalve for controlling the flow of fluid through a medical device, thevalve comprising a housing including a lumen extending therethrough anda flow control membrane extending across the lumen to control the flowof fluid through the lumen. The membrane includes a plurality of slitsextending therethrough so that, when the membrane is acted upon by apressure of at least a threshold magnitude, the slits open to permitflow through the lumen and, when not acted upon by a pressure of atleast the predetermined magnitude, the slits are maintained closed by abiasing force applied thereto by the membrane to prevent flow throughthe lumen. Each of the slits extends between end portions thereof alonga curve wherein a distance between a first end portion of a first one ofthe slits and a first end portion of a second one of the slits is aminimum distance between the first and second slits.

In another aspect, the present invention is directed to flow controldevice for a pressure actuated valve comprising a substantially planarelastic membrane including a peripheral seating portion adapted to besecured to a housing of the pressure actuated valve and a centralportion including a first curved slit extending therethrough. Theelastic membrane biases the first slit to a closed configuration inwhich edges of the first slit are in contact with one another to preventflow past the membrane so that, when the membrane is subject to apressure of at least a predetermined threshold magnitude, the membranemoves to an open configuration in which the edges of the first slit areseparated from one another so that fluid may flow past the membranethrough the first slit.

The present invention is directed to a dialysis catheter comprising acatheter body having a distal end insertable into a blood vessel, aproximal end connectable to a dialysis machine and a lumen extendingbetween the proximal and distal ends in combination with a pressureactuated valve disposed in the lumen to regulate flow therethrough andto seal the catheter when not in use. The valve includes a flow controlmembrane extending across the lumen, the membrane including a firstcurved slit extending therethrough, wherein, when the membrane is notsubject to a pressure of at least a predetermined threshold magnitude,the membrane is biased into a closed configuration in which edges of thefirst slit abut one another to prevent flow through the lumen and, whenthe membrane is subject to a pressure of at least a predeterminedthreshold magnitude, the membrane deforms to an open configuration inwhich edges of the first slit separate from one another to all flowthrough the lumen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a portion of a central line catheteraccording to an embodiment of the present invention;

FIG. 2 is a diagram showing a cutaway view of a valve assembly includinga high flow pressure activated valve membrane according to an embodimentof the present invention with the valve member in an open, in-flowconfiguration;

FIG. 3 is a diagram showing a cutaway view of the valve assembly of FIG.2 with the valve membrane in a closed configuration;

FIG. 4 is a diagram showing a cutaway view of the valve assembly of FIG.2 with the valve membrane in an open, out-flow configuration;

FIG. 5 is a diagram showing a silicone disk forming a high flow openableelement of a pressure activated valve according to an embodiment of thepresent invention;

FIG. 6 is a diagram showing a silicone disk forming a high flow openableelement of a pressure activated valve according to a second embodimentof the present invention;

FIG. 7 is a diagram showing a silicone disk forming a high flow openableelement of a pressure activated valve according to a third embodiment ofthe present invention;

FIG. 8 is a diagram showing a silicone disk forming a high flow openableelement of a pressure activated valve according to a fourth embodimentof the present invention; and

FIG. 9 is a diagram showing a silicone disk forming a high flow openableelement of a pressure activated valve according to a fifth embodiment ofthe present invention.

DETAILED DESCRIPTION

The present invention may be further understood with reference to thefollowing description and the appended drawings, wherein like elementsare referred to with the same reference numerals. The present inventionis related to medical devices that are used to access the vascularsystem of a patient, and in particular to central line catheters usedfor chronic access to a vein or artery. Although, embodiments of theinvention are described in regard to high flow valves for use in centralline catheters, those skilled in the art will understand that valvesaccording to the present invention may be employed to control flowthrough any device used to regularly access a body lumen.

Semi-permanently placed catheters may be useful for a variety of medicalprocedures which require repeated access to a patient's vascular systemin addition to the dialysis treatments mentioned above. For example,chemotherapy infusions may be repeated several times a week for extendedperiods of time. For safety reasons, as well as to improve the comfortof the patient, injections of these therapeutic agents may be bettercarried out with an implantable, semi-permanent vascular accesscatheter. Many other conditions that require chronic venous supply oftherapeutic agents, nutrients, blood products or other fluids to thepatient may also benefit from implantable access catheters, to avoidrepeated insertion of a needle into the patient's blood vessels. Thus,although the following description focuses on dialysis, those skilled inthe art will understand that the invention may be used in conjunctionwith any of a wide variety of procedures which require long termimplantation of catheters within the body.

Examples of such implantable catheters include those manufactured byVaxcel™, such as the Chronic Dialysis Catheter and the ImplantableVascular Access System. These devices typically are inserted under thepatient's skin, and have a distal end which includes a needle used toenter a blood vessel. The devices also have a proximal end extendingoutside the body for connection with an outside line. Thesesemi-permanent catheters may be sutured to the patient's skin tomaintain them in place while the patient goes about his or her normaloccupations.

FIG. 1 shows an exemplary catheter such as, for example, the Vaxcel™Chronic Dialysis Catheter. The catheter 10 has a distal end 12 that isinsertable into a patient's vein, and which remains within the patient'sbody for the life of the catheter 10. The distal end 12 includes aneedle (not shown) that pierces the vein of the patient to reach theflow of blood. During dialysis, blood from the patient is removedthrough the catheter 10, and is purified by a dialysis machine (notshown) which is connected to a hub 18 of the catheter 10 via an externalline 14. The catheter 10 may include two or more lumens with a first oneof the lumens being used to remove blood from the blood vessel and asecond one of the lumens being used to reintroduced treated blood and/ortherapeutic agents into the blood vessel. As described above, inaddition to dialysis, devices similar to the catheter 10 may be used toaccess a patient's vascular system for other types of treatment, forexample to infuse chemotherapy agents or other medications, to supplyfood and to remove blood samples.

When disconnected from the dialysis machine, the catheter 10 remainswithin the patient, connected to the patient's vascular system. Thus, itis important to securely seal the hub 18 to prevent fluids from escapingtherefrom and contaminants from entering the patient's body. Forexample, although the proximal end of the catheter 10 may be clamped toclose it off, if an effective seal is not obtained, the patient runs aserious of infection as well as risks of embolisms due to air enteringthe blood stream and venous thrombosis due to coagulation of blood inand near the catheter. In addition, leakage from an improperly sealedcatheter may expose attending medical staff to a risk of infection byblood borne pathogens. Thus a mechanism is necessary to ensure that thecatheter 10 is sealed when not in use.

Conventional clamps or clips have been used to seal such catheters 10between medical sessions. However, as the sealing forces repeatedlyapplied by these clips is exerted on a small portion of the surface areaof the catheter 10, damage to the wall of the catheter 10 at thisportion can significantly reduce the effective life of the catheter 10.It is also desired to improve the resistance of a sealing mechanism forthe catheter 10 to forces applied during activities of the patient, sothat the sealing mechanism will remain effective without restricting theactivity of the patient. Finally, it is desired to minimize the bulk ofthe sealing mechanism to enhance patient comfort.

An alternative to clamping or clipping the catheter 10 is to includeself sealing valves near the entrance of the flow passages of thecatheter, to seal those passages when not in use. For example, the hub18 may house one or more valve assemblies 20 which are designed to sealthe lumen(s) of the catheter 10 under certain conditions, and to allowpassage of fluid therethrough under other conditions. In an exemplarycase applicable to a dialysis catheter, the system of valves may sealthe catheter 10 when it is not connected to an operating dialysismachine, and may allow both an outflow of non-purified blood and aninflow of purified blood to the patient when an operating dialysismachine is connected thereto. These valve assemblies 20 thus selectivelyallow flow into or out of the patient only under predeterminedconditions when they are placed in fluid contact with the inflow oroutflow portions of a dialysis catheter 10.

Pressure activated safety valves (PASV's) are one type of flow controldevice that has been used to seal vascular catheters when not in use.These valves open when subject to flow pressure of at least apre-determined value and remain closed when subject to pressures belowthe pre-determined value. In the exemplary case of a PASV used in adialysis catheter, the valve is preferably designed so that thepre-determined pressure substantially exceeds a pressure to which thevalve would be subjected from the vascular system or due to patientactivity and may correspond to a pressure approximating a lower level ofthe pressures to which the valve would be subjected by an operatingdialysis machine. Thus, when no dialysis machine is connected to thecatheter, the pressure in the lumen is insufficient to open the PASV,and the catheter remains sealed.

FIGS. 2-4 show more detailed views of a PASV assembly 20 in a cutawaydrawing depicting three flow conditions. FIG. 2 shows a configuration ofthe assembly 20 in which a fluid is being introduced into catheter 10via a hub 18 while FIG. 4 shows a configuration of the assembly 20 inwhich a fluid is being removed from the catheter 10 to the hub 18. FIG.3 shows a configuration of the assembly 20 in a closed configuration inwhich flow therethrough is prevented. In the context of a dialysiscatheter, the configurations of FIGS. 2 and 4 correspond, respectively,to blood being returned to and being withdrawn from a patient. Theconfiguration of FIG. 3 corresponds to a condition in which no dialysistreatment is being performed, or in which a treatment has beentemporarily halted so that the assembly 20 seals a lumen of the catheter10. According to one exemplary embodiment of the present invention, thevalve assembly 20 comprises a valve housing 30 forming a body of thedevice and a slitted membrane 32 disposed within the housing 30. The hub18 may define the valve housing 30 or, alternatively, the housing 30 andthe hub 18 may be formed as separate units. The housing 30 defines aflow chamber 36 through which fluid (e.g., blood) flows into and out ofthe catheter 10. The exemplary flow chamber 36 is substantiallycylindrical. However in different applications, the flow chamber 36 maybe of any other shape suitable for the efficient flow of a fluidtherethrough.

The slitted membrane 32 may be disposed at one end of the flow chamber36, and is positioned to selectively impede the passage of fluid thoughthe flow chamber 36. A curved slit 34 is formed in the membrane 32 sothat, only under predetermined conditions, the slit 34 is opened topermit fluid flow through the flow chamber 36. When the membrane 32 isnot exposed to the predetermined conditions, the slit 34 remains closedto seal the flow chamber 36. For example, the slitted membrane 32 may beconstructed so that the curved slit 34 opens only when subject to a flowpressure of at least a threshold magnitude. When a pressure to which theslitted membrane 32 is subject is less than this threshold pressure, theslit 34 remains closed. The threshold pressure may correspond, forexample, to the pressure generated in the flow chamber 36 when thecatheter 10 is coupled to an operating dialysis machine. In addition,the membrane 32 is preferably constructed so that the threshold pressureis significantly greater than pressures which will be generated withinthe catheter 10 by the vascular system or due to activities of thepatient.

FIGS. 2-4 show one exemplary embodiment of a pressure activated valveassembly 20 according to the present invention. Those of skill in theart will understand that different configurations of the housing 30, theslitted membrane 32 and the slit 34 may be used without departing fromthe invention. For example, the membrane 32 may include one or moreslits of various sizes and shapes to tailor the flow through membrane 32and to vary the threshold pressure required to open slit 34. Thoseskilled in the art will understand that the shape of the membrane 32 andits placement within the housing 30 may also be varied to accommodatedifferent designs of the housing 30.

Pressure actuated valve membranes which seal catheters when not in usehave often relied on limitations in the size of the slits therethroughto ensure complete closure of the slits when not subject to at least athreshold pressure. However, this may also limit the flow rate that maybe obtained through the valve membrane. Thus, it is important to ensurecomplete sealing of the catheter 10 while permitting an increased flowrate to allow treatment sessions to be shortened.

The effective area of the opening in the valve membrane 32 is afunction, among other things, of the length and width of the slit 34, aswell as the stiffness of the material forming the membrane 32. In thecase of a very stiff membrane 32, the opening area is determinedprimarily by the length and width of the slit 34. When the membrane 32is flexible, the effective opening area also varies based on a degree towhich the edges of the slit 34 bend away from a plane in which themembrane 32 resides when closed. This bending essentially forms flaps ofmembrane material deflected in direction of the flow therethrough. Amore flexible membrane 32 therefore allows a larger opening area and,consequently, permits more flow to pass therethrough for a given size ofthe slit (or slits) 34 formed therein. One drawback of increasedflexibility in such membranes is that the material may not havesufficient resilience to maintain the slit 34 closed when necessary.Thus, in order to ensure that the valve assembly 20 is effective,additional stiffeners may be required thereby increasing the complexityand cost of manufacture of the assembly 20.

According to exemplary embodiments of the present invention, the flowrate through a PASV assembly is increased by utilizing one or morecurved slits having a specified radius of curvature as opposed to linearslits. Those skilled in the art will recognize that the curves alongwhich the slits extend may be of any non-linear shape to increase thelength of the curve relative to that of a line drawn between end pointsof the curve. In addition, for slits extending substantially parallel toone another (i.e., where lines drawn between end points of the slitswould be parallel), end points of the curves will preferably be thepoints of closest approach of the slits to one another. In this manner,the material of which the membrane is formed may be selected withsufficient stiffness to retain the slit(s) completely closed when themembrane is not subjected to a pressure of at least the thresholdmagnitude, without excessively reducing the flow of fluid therethrough.Although the endpoints of the slit may be the same for linear and curvedslits, the curved slit has a greater length which can be computed basedon the radius of curvature of the slit. It may be beneficial to utilizeendpoints for the curved slits which are the same as would have beenused for linear slit, for example, to respect structural constraintsnecessary to the integrity of the membrane. The arc length of the slitbetween the endpoints can be easily computed using simple geometricformulas. For example, if the slits were along an arc having a radius of0.59 mm and an angle of 59°, the curve length and the correspondingstraight line length between the endpoints would be as follows:

$\begin{matrix}{{{curve}\mspace{14mu}{length}} = {\left( {{radius}*{angle}*\Pi} \right)/180}} \\{= {\left( {0.59\mspace{14mu}{mm}*59*\Pi} \right)/180}} \\{= {0.607\mspace{14mu}{mm}}}\end{matrix}$ $\begin{matrix}{{{straight}\mspace{14mu}{line}\mspace{14mu}{length}} = {{Sin}\mspace{14mu}\left( {{angle}/2} \right)*2\mspace{14mu}{radius}}} \\{= {{Sin}\mspace{14mu}\left( {59/2} \right)*2*0.59\mspace{14mu}{mm}}} \\{= {0.581\mspace{14mu}{mm}}}\end{matrix}$Alternatively as described in the Machinery's Handbook (22^(nd) Edition)from Industrial Press, the straight line length may be found using theformula:straight line length=2*(h*(2*radius−h))^(1/2)where h is the height from the center of the straight line to the arc.Thus, the area of the opening for a curved slit is greater than for alinear slit because of the greater arc length of the curved slit, andbecause the flap formed on the concave side of the curved slit is betterable to deflect with the flow, since it is unconstrained along a greaterlength.

FIG. 5 shows an exemplary embodiment of a flow control membrane 100having two curved slits 104, 106 located in a central portion of themembrane 100. The membrane 100 comprises a peripheral portion 102adapted to be held in place within a housing of a pressure actuatedvalve, for example, by compression between two halves of the housing 30clamped therearound. The membrane 100 also includes a central portion101 that extends across a lumen of the housing 30 (e.g., in a flowchamber 36 thereof to selectively permit and prevent flow therethrough.The curved slit 104 extends across a portion of the central portion 101between edges 108 and 110 while the curved slit 106 extends across aportion of the central portion 101 between edges 112 and 114. The edges108 and 110 and 112 and 114 are biased to remain joined to one anotherin the closed configuration to prevent flow through the membrane 100whenever the membrane 100 is not subject to a pressure at least as greatas the threshold pressure.

As discussed above, in the open configuration, the curved slits 104, 106form a larger opening area than would be formed by linear slitsextending between the same end points. That is, the arc length betweenthe endpoints 116, 118 is greater than a straight line between those twoendpoints. In addition, the regions of the membrane 100 between theconcave sides of the slits 104, 106 forms a flap 120 which, when theedges 108 and 110 and 112 and 114 separate from one another, uncovers alarge opening area. Specifically, the edges of the flap 120 adjacent tothe edges 110 and 112 of the slits 104 and 106, respectively, areunconstrained along a greater length than would be the case for acorresponding pair of linear slits extending between the end points 116and 118. In different embodiments, only one curved slit may be used, oradditional slits may be formed in the membrane, depending on the designrequirements of the valve and, in particular, on the threshold pressureand flow rate values sought. Those of skill in the art will understandthat the flap will be greater on the side that is less supported eitherby the seat or by the lack of an adjacent slit.

In the exemplary embodiment, the curved slits 104, 106 are substantialmirror images of one another with curvatures substantially similar toone another. For example, the slit 104 extends along a portion of acircle with a radius of curvature 124 extending to a center of curvature126. In different embodiments, each of the slits 104, 106 may have anyor all of a different radius of curvature, a different orientation, or adifferent length. In this exemplary embodiment, the curved slits 104,106 are substantially symmetrical about a line of symmetry 122, which inthis case is a horizontal axis, or major axis of the elliptical membrane100. In other embodiments that include two or more curved slits, theslits may be disposed on the membrane in a configuration symmetricalwith respect to different lines, or with respect to a point, dependingon the flow requirements of the membrane. Alternatively, the slits maybe arranged asymmetrically with respect to one another.

An additional advantage of a membrane having curved slits is thatinterference between the slits can be minimized. When edges 108, 110 and112, 114 of slits 104, 106 separate, they cause a portion of themembrane adjoining the opening to fold outward, in the flow direction.In the case of linear slits, these folded over portions can interferewith each other, thus preventing a complete opening of the opening area.This is somewhat analogous to having two adjacent doors opening towardseach other with Interference between the open doors preventing a fullopening of either one. In the case of a membrane 100 with curved slits104,106, the curvature moves the edges 110 and 112 away from each other,reducing the possibility of adverse interference between open slits 104and 106. The reduced interference between curved slits may also furtherincrease the flow of fluid that can pass through the membrane 100, ascompared to a membrane having comparable linear slits. The flow rate maydepend on the particular application and can range from the handinjection flow rate of approximately 0.5 cc/minute to a maximum of 750ml/minute. To provide a specific example, the flow rate for dialysis mayrange from 250 ml/minute to 500 ml/minute, the preferable target flowrate being 275 ml/minute.

FIG. 6 shows a top elevation diagram of a second embodiment of a flowcontrol membrane having curved slits. In this case, the membrane 200 hasa central portion 201 which defines four curved slits 202, 204, 206 and208. The slits are distributed substantially symmetrically in pairsalong a line of symmetry 122, and have radii of curvature selected suchthat at least edges 210, 212 substantially approximate an adjacentportion of a periphery of the membrane 200. In this manner, an evengreater amount of fluid can flow through the membrane 200 when the slits202-212 are moved into the open configuration by fluid pressure. As withthe membrane 100 of FIG. 5, the curvature of the slits 202, 204, 206 and208 allows them to open without interfering with one another. Inaddition, as the outer edges of the slits 202 and 208 substantiallyparallel the outer periphery of the membrane 200, a minimum clearance214 between these slits and the outer edge of the membrane may bemaintained so that the structural integrity of the membrane 200 is notcompromised. For example, for a membrane having a thickness of 1 mm, theclearance may range from 0.005-0.040 inches.

If linear slits were to be substituted for these curved slits, it wouldbe difficult to achieve a corresponding slit length (or even acorresponding distance between slit end points) without compromising theminimum clearance 214 and without encountering resistance to openingthrough contact between the edges of adjacent slits. That is, the edgesof four such linear slits that had a length comparable to the curvedslits would be very close to each other, and would likely suffer frominterference effects. In addition, the endpoints of the outermost slitswould be disproportionally near the edges of the elliptic membrane,raising the possibility of tears of the membrane near the slits' edges.Accordingly, embodiments of the membrane 200 which use curved slits, asprescribed by the present invention, can include a greater number ofslits of greater length to obtain a greater flow rate while avoiding thedrawbacks normally associated with multiple slits in such valvemembranes.

Different lines of symmetry may be utilized to define slits in the flowcontrol membrane according to the present invention. For example, slits304 and 310 and slits 308 and 306 of the membrane 300 in FIG. 7 aresymmetrical to each other with respect to a vertical line of symmetry302. This configuration forms several sub-regions of the flow controlmembrane 300 which cooperate to define large opening areas through whichfluid can pass when the membrane 300 is in the open configuration. Forexample, edges 312 and 314 of the slits 304, 306, respectively, define aregion 320 that is displaced from the plane in which the membrane 300resides when in the closed position, as the edges of the two slitsseparate, into the open configuration. Similarly, edges 316 and 318 ofthe slits 306, 308 define the boundaries of a region 322 which moves outof the plane of the membrane 300 when the slit edges separate to theopen configuration. Symmetric regions 330 and 332 operate substantiallyin the same way.

In the exemplary configuration shown in FIG. 7, the slits 304, 306, 308and 310 are designed to complement each other in pairs, so that a largerflow opening area may be created when the membrane 300 is moved to theopen configuration. Other similar slit configurations may be obtained,for example by modifying the radii of curvature of the curved slits, orby positioning the slits differently, within the same or a similarpattern of symmetry. As described above, one constraint on thepositioning of the curved slits is that the outer edges of the slitsmaintain a sufficient minimum distance from the edge 324 of the membrane300 so that the structure of the membrane 300 is not compromised.

FIG. 8 shows a further exemplary embodiment of the flow control membraneaccording to the invention. Membrane 400 defines three curved slits 402,404 and 406 which diverge substantially radially from a central point412. Alternatively, a curved slit 410 may be used in place of the slit406, thus making the configuration symmetrical around a single point ofsymmetry (i.e., central point 412). In either of these configurations,the curved slits 402, 404 and 406 (or 410) cooperate to form a largeropening than might be calculated from a sum of their individualcontributions. In the open configuration, portion 414 of the membrane400 is displaced in the direction of fluid flow out of the plane inwhich the membrane 400 resides when in the closed position, as edges ofthe curved slits separate from one another. This out-of-planedisplacement causes a large opening area for the flow to be formed. Forexample, if slits 410, 404 and 402 are used, the flow will be aspiraling flow causing the reversing of one of the slits such as slit406. This action will cause a counter flow with slit 404 and allow abroader stream.

FIG. 9 shows a further exemplary embodiment of the flow control membraneaccording to the invention. Membrane 500 defines an S-shaped slit 506that is symmetrical about the bisecting lines 502 and 504.Alternatively, the single S-shaped slit 506 may be broken into severalsmaller curved slits along the same path as the S-shaped slit 506, e.g.,multiple slits forming a broken S-shape. As can be seen from FIG. 9, theS-shaped slit 506 is a series of curved slits in the membrane 500allowing for a substantially greater length than a straight slit fromendpoint 508 to endpoint 510. Likewise, the described broken S-shape(not shown) will also have a substantially greater length than astraight slit from endpoints 508 to 510.

It should be noted that the above described examples includedconfigurations having multiple slits. It is possible to design themembranes such that the slits open in a pre-determined order. Forexample, the slits may open one at a time or may be staged in anydesired configuration. The varied opening times and order may be basedon the pressure build up due to the flow through the various slits. Thepressure sensitive slits may be created using local variations in themembrane thickness or by varying the size and/or placement of the slitsin the membrane.

The present invention has been described with reference to specificembodiments, more specifically to a pressure activated safety valve usedin a dialysis catheter. However, other embodiments may be devised thatare applicable to other medical devices, without departing from thescope of the invention. Accordingly, various modifications and changesmay be made to the embodiments without departing from the broadestspirit and scope of the present invention as set forth in the claimsthat follow. The specification and drawings are accordingly to beregarded in an illustrative rather than a restrictive sense.

1. A valve for a medical device, comprising: a housing including a lumenextending therethrough; and a substantially planar membrane extendingacross the lumen to control the flow of fluid through the lumen, themembrane including first and second curved slits disposed symmetricallyabout an axis of the membrane and at least one additional slit disposedin between said first and second curved slits.
 2. The valve of claim 1,wherein said first and second curved slits have curvatures thatapproximate adjacent portions of a periphery of said membrane.
 3. Thevalve of claim 1, wherein the slits open when acted upon by a pressureof at least a threshold magnitude.
 4. The valve of claim 1, wherein themembrane is elliptical and the first and second slits are disposedsymmetrically about a major axis of the membrane.
 5. A flow controldevice for a pressure actuated valve, comprising: a substantially planarelastic membrane including a peripheral seating portion adapted to besecured to a housing of the pressure actuated valve and a centralportion including first and second curved slits disposed symmetricallyabout an axis of the membrane and at least one additional slit disposedin between said first and second curved slits.
 6. The flow controldevice of claim 5, wherein said first and second curved slits havecurvatures that approximate adjacent portions of a periphery of saidmembrane.
 7. The flow control device of claim 5, wherein the slits openwhen acted upon by a pressure of at least a threshold magnitude.
 8. Theflow control device of claim 5, wherein the membrane is elliptical andthe first and second slits are disposed symmetrically about a major axisof the membrane.
 9. A dialysis catheter, comprising: a catheter bodyhaving a distal end insertable into a blood vessel, a proximal endconnectable to a dialysis machine and a lumen extending between theproximal and distal ends; and a pressure actuated valve disposed in thelumen to regulate flow therethrough, the valve comprising: asubstantially planar elastic membrane including first and second curvedslits disposed symmetrically about the membrane and at least oneadditional slit disposed in between said first and second curved slits.10. The dialysis catheter of claim 9, wherein said first and secondcurved slits have curvatures that approximate adjacent portions of aperiphery of said membrane.
 11. The dialysis catheter of claim 9,wherein the slits open when acted upon by a pressure of at least athreshold magnitude.
 12. The dialysis catheter of claim 9, wherein theslits open when acted upon by a pressure of at least a thresholdmagnitude, said threshold being substantially greater than would beinduced by a patient's vascular system.
 13. The dialysis catheter ofclaim 9, wherein the membrane is elliptical and the first and secondslits are disposed symmetrically about a major axis of the membrane.